Until the advent of Coplanar Waveguides (CPWs), microstrip line was the conventional broadband transmission medium employed for use in electronics operating at the microwave and millimeter wave frequency bands. However, the major drawback with microstrip line is the difficulty encountered in placing series and shunt components on the same surface as the microstrip signal conductor. The problem arises because the ground conductor—to which electrical contact is essential for the operation of many components—is conventionally formed on the backside of a substrate (e.g. Duroid, ceramic, etc.) on which the microstrip line is formed. Consequently, conductor-filled via holes through the substrate must be made to connect components on the topside of the substrate with the ground conductor (i.e. ground plane) on the bottom side of the substrate. The conducting material used in the via holes adds parasitics, such as unwanted inductances and resistances, to circuits assembled on the top side of the substrate. The parasitics in many cases lead to limits on the high frequency performance of the microstrip lines and circuits that include them.
CPWs, on the other hand, are better suited for high frequency and ultra broadband transmission applications since their basic structure is one in which both the signal and ground conductors lie on a same plane. Conventionally, a CPW includes a central signal conductor and two ground conductors arranged to form a Ground-Signal-Ground pattern with all three conductors lying in a same plane. The signal conductor is, of course, not physically (i.e. electrically) contacting either of the two ground conductors; however the respective spacing between the signal conductor and either of the two ground conductors is made close enough that the signal conductor is electromagnetically coupled to both ground conductors. The signal conductor and two ground conductors of a CPW are typically mounted on the flat top surface of a substrate that defines the plane of the CPW. It is not uncommon for the flat under side of the substrate to be covered by a conductive metal thus forming a Conductor-Backed CPW (CBCPW).
Ideal CPW transmission lines would have expansive substrates and ground planes. However, such a structure is impractical to construct. Accordingly, conventional substrates used have a finite thickness (and width) and each of the ground conductors must also have a finite width. Beyond these two approximations, other refinements can be made in order to tailor the performance of the CPW structure so that CPWs may be integrated into various microwave or millimeter wave circuits and assemblies (e.g. packages).
Specifically, CPWs of a wide range of impedances can be synthesized by varying the signal conductor and slot (gap) width(s). A slot width is the distance between the signal conductor and a respective ground conductor. With two degrees of freedom (signal conductor and gap widths), as compared to microstrip line which has only one degree of freedom for a given substrate thickness, CPWs can accommodate components without the added worry of compromising the CPWs characteristic impedance during assembly of a circuit. Moreover, ground return paths and connections can be kept very short for a CPW to afford good broadband high frequency performance.
The disadvantages of CPWs include the higher possibility of dominant undesired mode generation and lower power handling capability as compared to other available transmission media in the frequency bands of interest. There is especially a problem with spurious mode (i.e. unwanted electromagnetic wave modes) generation associated with broadband signal transmission on Conductor-Backed Coplanar Waveguides (CBCPW).
CBCPWs support modes which can be categorized into one of three groups: 1) transmission modes guided by the CPW slots (gaps)—of which there is usually just one known as the fundamental mode, which is utilized for the transmission of signals on the CPW; 2) parallel-plate modes guided between the CPW plane and the backside conducting plane; and 3) possible parallel-plate modes guided in the space between a cover (above the CPW plane) and the signal conductor. The third group of modes (possible parallel plate modes) is relatively less important since the cover can usually be moved far enough away from the top for CPW to avoid the unwanted effects. Of the second group, only the lowest order mode is usually present but the second group serves as a detrimental vehicle for energy leakage from the fundamental mode supported by the CPW. Leakage occurs when the phase velocity of the parasitic mode(s) is slower than the phase velocity of the fundamental mode. Generally, the leakage is a continuous function of frequency with a leakage angle that varies such that the parasitic mode phase velocity projected along the fundamental mode direction matches the fundamental mode phase velocity. In a conductor-backed CPW, the backside conductive plane parallel-plate mode is generally slower than the fundamental CPW mode (in terms of phase velocity) and thus energy leakage occurs at all frequencies.
From a time-domain perspective, wideband signals typically consist of pulses of a few picoseconds in duration that need to be transmitted with a high-fidelity pulse shape which is faithfully maintained in the transmission medium through to the receive (Rx) end. If the high-fidelity of the pulses is not maintained, consecutively transmitted pulses smear into one another leading to a phenomenon known as Inter-Symbol Interference (ISI). Unfortunately, currently known CPW structures do not provide much freedom of design that can be taken advantage of to significantly lower the effects of ISI.
For example, in OC768 based systems, 40 Gbps opto-electronic networks require undistorted transmission of picosecond pulses over optical and electronic transmission media. Compared to the generation and characterization of picosecond electrical pulses, which is an almost fully matured technology, the development of transmission structures capable of handling the extremely wide bandwidth of these pulses still remains difficult. For electrical pulses a few tens of picoseconds in duration, modal dispersion due to the physical dimensions (i.e. geometry) of the transmission media is the dominant factor contributing to pulse distortion.
Illustrated in FIG. 1 is a cross-sectional view of a prior art Coplanar Waveguide (CPW) structure 100. The prior art CPW structure 100 is comprised of a signal conductor 20 and ground conductors 22 and 23 spaced away from either side of the signal conductor 20 respective lateral distances s1 and s2. The signal conductor 20 and ground conductors 22 and 23 are all on a same plane that is defined by the top surface of the substrate 30, which rests atop a surface of a package base 50 and inherently has a dielectric constant. The surface of the package base 50 (or the entire package base 50) is conductive so that the surface of the (entire) package base 50 or the entire package base can be biased at and thus provide the ground potential for equipment in which the substrate is incorporated. Lastly, the prior CPW structure 100 may optionally include a conductive back plate 40 affixed between the bottom of the substrate 30 and the surface of the package base 50.